Impregnated porous photoconductive device and method of manufacture

ABSTRACT

Impregnating a porous photoconductive device with molten glass, or some other physically and chemically stable substance, to make the dark resistance more predictable and to increase the charge retentivity many times.

Umted States Patent 1 1 [111 3,745,504 Mooney 1 1 July 10, 1973 IMPREGNATED POROUS 2,937,353 5/1960 Wasserman 338/15 PHQTOCONDUCTIVE DEVICE AND 3,248,261 4/1966 Narken et a1. 338/15 METHOD OF MANUFACTURE FOREIGN PATENTS OR APPLICATIONS 1,400,552 4/1965 France 338/15 Primary Examiner-T. H. Tubbesing Attorney-Lowhurst & Hamrick 5 7 ABSTRACT impregnating a porous photoconductive device with molten glass, or some other physically and chemically stable substance, to make the dark resistance more predictable and to increase the charge retentivity many times.

14 Claims, 5 Drawing Figures GLASS 1M PREGNATED PHOTOCONDUCTIVE LAYER PAIENIEU JUL 1 0 ms POWDERED GLASS SINTERED PHOTOCONDUCTIVE LAYER Fig-2 Fig-1 GLASS IMPREGNATED PHOTOCONDUCTIVE LAYER I I I I MOLTEN GLASS INVENTOR JOHN B. MOONEY ATTORNEYS IMPREGNATED POROUS PI-IOTOCONDUCTIVE DEVICE AND METHOD OF MANUFACTURE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to photoconductive devices and, more particularly, to a method of making photoconducting layers with a predictable dark resistance and a substantially increased charge retentivity.

2. Discussion of the Prior Art A photoconductive device is generally comprised of a body of photoconductive material disposed between a pair of electrodes which has the characteristic that with a voltage applied across the electrodes, the device displays a decrease in electrical resistance proportional to the intensity of sensitive radiation incident on the photoconductive material. In other words, the amount of current flowing through the device is a function of its electrical resistance, and the electrical resistance is a function of the incident radiation. The ideal photoconductor has infinite resistance in the absence of radiation and zero resistance in the presence of a maximum radiation to which it is sensitive.

In practice, however, a photoconductive device behaves as a high resistance conductor when the radiation to which it is sensitive is absent, and as a low resistance conductor when the radiation to which the device is sensitive is present. The current passed by the device in the absence of radiation, which is usually light in the visible or immediately adjacent portion of the electromagnetic spectrum, is typically referred to as the dark current, while the current passed when the device is irradiated is referred to as the light current. The difference between light current and dark current is usually referred to as the photocurrent.

Although there are several types of photoconductive devices which exhibit the above-mentioned characteristics, one type which is of particular interest herein is the sintered photoconductor which is generally made by sintering a mixture of a granulated photoconductive material, with or without a suitable binder, to form a porous polycrystalline layer of interlocked photoconducting crystals. Such devices are disclosed in the US. Pat to Thomsen Nos. 2,765,385; J. G. Van Santen et al. 2,930,999; Schonebarger, 2,993,001; I-Iushley 2,689,188; and Wasserman 2,937,353.

Sintered photoconductive devices are typically porous in character and thus have gases entrained within their pores. The presence of these gases within the device tends to limit the dark resistance of the device and also effects the predictability of the dark resistance. Although the reason why this resistance limiting and unpredictability occurs is not yet fully understood, it has been found that by removing the gases, the dark resistance of the photoconducting layer can be substantially improved. One possible explanation is that a gas discharge occurs within the pores which provides electrical shorting between the electrodes as well as between the various sintered particles, and that the material forming the pore walls is affected by the entrained atmospheric gases through a chemical or physical reactron.

SUMMARY OF THE PRESENT INVENTION It is therefore an object of the present invention to provide a method of manufacturing a sintered photoconductive device having a predictable dark resistance.

Another object of the present invention is to provide a method of purging gases from the pores of a sintered photoconductive material to thereby improve the operational characteristics of the device.

Another object of the present invention is to provide a method of increasing the dark resistance of a sintered photoconductive device without materially affecting the luminous sensitivity thereof.

Another object of the present invention is to protect a porous photoconductive member from deterioration due to chemical or physical effects of the entrained ambient atmosphere on the pore surfaces and adjacent material.

The above objects are accomplished in accordance with the present invention by impregnating a porous sintered photoconductive member with a predetermined quantity of molten glass which, in flowing through the pores of the member, displaces the entrained gases and, upon solidifying, forms a solid structure substantially free of gas containing pockets. As a result, a photoconductive device is provided which has a higher dark conductivity than prior art devices with similar photoconductive compositions while having luminous sensitivity that is substantially unchanged and is predictable.

The advantages of the present invention will no doubt become apparent after a reading of the following detailed disclosure whichmakes reference to the several figures of the drawings.

IN THE DRAWING FIGS. 1-4 are cross-sectional views showing different stages in the manufacturing of the impregnated photoconductive device of this invention; and

FIG. 5 is a cross-sectional view illustrating a photosensitive capacitor made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The process for preparing the photoconductive device of the present invention utilizes a conventional sintered layer 10 of photoconductive material, such as illustrated in FIG. 1. Layer 10 is comprised of a plurality of photoconductive crystals 12 fused together by sintering which causes the layer to be porous.

One method of producing a suitable photoconductive layer is to form a stratum, including particles 12, of a material selected from the group consisting of sulphides, selenides and sulphoselenides of cadmium, recrystallizing the material in a molten solvent to a desired range of particle sizes, incorporating into the recrystallized material activator proportions of a halide and activator porportions of a metal selected from the group consisting of copper and silver, and then evaporating the molten solvent, thereby producing a substantially continuous, although porous, layer of interlocked crystals of photoconductive material. This method is described in more detail in the aforementioned Thomsen patent, but it should be understood that the present invention is applicable to all porous photoconductive layers or members, regardless of the method used to produce it.

Although sintered photoconductive layer 10 typically exhibits good dark resistivity characteristics, it has been found that such resistivity can be improved in accordance with the present invention. Even though crystals 12 are fused together at their points of contact 18 during the sintering operation, it can be seen that a large plurality of interstices or pores 20 remain therebetween leaving the sintered mass with a porous character. And since pores 20 contain small quantities of gas, it is theorized that the dark resistance of the device is limited by the occurrence of spurious electrical discharge paths created between the crystals 12 due to the particle heating and the electric fields developed by the current flowing through the particulate mass. If this phenomenon does, in fact, occur, the resulting effect would apparently be to limit the dark resistance of the device, and accordingly, to eliminate such discharge paths should have the effect of improving the dark resistance. As it turns out, the dark resistance does increase when the gas is removed and this tends to indicate support for this theory.

In order to purge sintered layer 10, typically having an electrode 16, of gases entrained in pores 20, a quantity of powdered solder glass 22, or other electrically nonconductive material which is substantially inert to the atmosphere and the photoconductive material, is deposited on the upper surface 24 of porous layer 10. Where powdered solder glass is utilized, it can be changed into its liquid phase by simply heating it to above its melting point for a short period of time (for example, 600C for three minutes). Once the glass assumes its liquid phase, as illustrated in FIG. 3, the gases contained within pores 20 will be displaced as the glass flows into the pores.

The quantity of glass 22 deposited on surface 24 is preferably chosen, for maximum effect, to be sufficient to completely fill the pores of layer 10 without leaving a substantial residue over surface 24, thus eliminating the need to abrade or polish away the excess glass so that electrical contact can be made with surface 24. After layer 10 is impregnated, a suitable optically transparent metallic electrode 28 may be provided upon surface 24 as illustrated in FIG. 4, or for electrostatic ap plications, surface 24 may be left exposed. In the case of impregnating a lateral structure, in which the electrodes make contact with the edges of a layer of photoconductive material, there is no need to remove excess glass so that the major criteria is to select sufficient glass to fill the pores.

It will be appreciated that since the improvement in resistance is obtained by eliminating possible gas discharge paths, there will be a direct relationship between the amount of glass introduced into the device and the dark resistivity. This, of course, means that the dark resistivity of a given sintered device can be controlled by selecting the quantity of glass to be introduced into pores 20. In other words, by filling less than all pores 20, a smaller improvement in the dark resistance is obtained. In any event, the resultant photoconductivevlayer will have a substantially increased dark resistance as compared with the photoconductive layer illustrated in FIG. 1, even though the photoconductive response or photocurrent will be practically unchanged.

The impregnated photoconductive material can be con-sidered a matrix of photoconductive material sintered to produce good grain-to-grain contact, and the glass impregnant provides stabilization of the pore and other surfaces and superior strength to the layer.

While glass is a preferred impregnant, it should be understood that any other material, capable of being flown into the pores, can be used providing the material is substantially inert with respect to the photoconductive material and the environment, such as ambient atmosphere. Further, the impregnant can, but need not necessarily, be transparent to the radiation to which the photoconductive material is sensitive. For example, in the case of a photoconductive material sensitive to frequencies lying in the visible light spectrum, the impregnant is preferably clear transparent, particularly when the pores are to be entirely filled to allow the sensitive radiation to reach the photoconductive or photosensitive material. On the other hand, if the pores are only to be partically filled, the impregnant may be opaque to the radiation since there would be no or little interference with radiation reaching the sensitive material.

In some applications, such as Xerography, it is desirable to have a large electrostatic field which requires a high dark resistivity for high charge retentivity. It has been found that impregnation of the photoconductive layer does provide a con-siderably increased dark resistivity without materially affecting the sensitivity which is defined herein as the percentage of charge retention of light to dark condition of the photoconductive material.

As a practical matter, the sintered photoconductive material is selected to provide a desired or required sensitivity. It is then impregnated in accordance with the present invention which increases the dark resistivity, and thereby the charge retention, without materially affecting the sensitivity. The following table illustrates two samples and provides data on the retained charge in the case of unimpregnated and impregnated samples for the dark and the light condition.

The charge retention of the photoconductive layers set forth in the Dark and Light columns are respectively for the dark and after a 2 lux-second exposure to light. As indicated, the samples were tested before and after impregnation with Corning 7570 solder glass. The two photoconductive layer samples were prepared in the ordinary manner, and only differ with respect to the details as to doping which account for the differences. Both samples were impregnated as explained above and in the same manner. These samples show that the sensitivity in terms of the degree of discharge, expressed as percent of the dark charge retention, is substantially unchanged by impregnation while the charge retention in the dark is increased in these examples by a factor of about 4 to 6.

While the amount of impregnant deposited on top of the sintered photoconductive layer, for sandwich type photoconductive devices, is selected to prevent the formation of a glass layer over the surface, there are other applications in which an insu-lating layer is desired or required. One of such applications is in the construction of capacitive impedances which employ a photosensitive dielectric whose dielectric constant changes with changes in the radiation to which the dielectric is sensitive. It has been found that photoconductive materials of the type referred to herein not only change their resistance as a function of sensitive radiation, but also their dielectric constant.

Referring now to FIG. 5, there is shown a photosensitive capacitor constructed in accordance with the present invention. A sintered layer of photoconductive material 30, composed of particles 31, is provided with an electrode 32 which may be a metallic film deposited on a substrate 34. Material 30 is then impregnated with an impregnant 36 of which there is sufficient to form an insulating layer 38 on the top of photoconductive material 30. A further electrode 40, which must be radiation transparent for the sandwich construction illustrated in FIG. 5, is placed adjacent the top surface of insulation layer 38, and a photosensitive capacitor is formed.

It is contemplated that certain modifications of the present invention will become apparent to those skilled in the art after having read the above disclosure of the preferred embodiment. It is therefore intended that the appended claims be interpreted as covering all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A method of increasing the dark resistivity of a porous sintered photoconductive element, comprising the step of impregnating the pores of said photoconductive element with a glass impregnant by heating the impregnant to its melting point and flowing it into the pores of said photoconductive element.

2. A method in accordance with claim 1 in which substantially the pores are fully impregnated.

3. A method in accordance with claim 1 in which the pores are partially impregnated leaving some pore space unimpregnated.

4. A method of manufacturing a photoconductive device comprising the steps of:

depositing a predetermined quantity of solid glass impregnant upon a porous mass of sintered photoconductive material; and

heating said photoconductive material and said impregnant to the melting point of said impregnant for flowing the molten impregnant into the pores of said porous mass.

5. A method of manufacturing a sintered photoconductive device comprising the steps of:

depositing a mixture including a particulate photoconductive material upon a first metallic electrode; heating said mixture to a first temperature to sinter said mixture to form a sintered photoconductive element having said first electrode in conductive contact with one side thereof; depositing a predetermined quantity of glass upon said sintered photoconductive element; and heating said glass and said sintered photoconductive element to a second temperature sufficient to melt said glass for flowing the molten glass into the pores of said sintered photoconductive element.

6. A method of manufacturing a sintered photoconductive device as recited in claim 5 which further includes the step of providing a second metallic electrode in electrical contact with the other side of said sintered photoconductive element.

7. A method of manufacturing a sintered photoconductive device as recited in claim 5 wherein the quantity of glass de-posited on the sintered photoconductive element is in excess of the quantity required to completely fill the pores of said element and which, upon being cooled, forms a layer of glass of a predetermined thickness upon said element and which further includes the step of providing a second metallic electrode upon said layer of glass, said layer of glass electrically isolating said second electrode from the other side of said element.

8. A sintered photoconductive device comprising:

a first metallic electrode;

a sintered photoconductive element disposed in electrical contact with said first electrode; and

an inorganic electrically nonconductive, solid impregnant disposed in the pores of said element.

9. A sintered photoconductive device as recited in claim 8 which further comprises a second metallic electrode in electrical contact with said member and spaced apart from said first electrode.

10. In a photoconductive device, including a sintered photoconductive element having pores, the improvement comprising an inorganic, electrically nonconductive, solid impregnant disposed in said pores.

11. In a photoconductive device in accordance with claim 10 in which said impregnant is transparent to the radia-tion to which said element is sensitive.

12. In a photoconductive device in accordance with claim 10 in which said impregnant is substantially inert to the ambient atmosphere and to said element.

13. In a photoconductive device in accordance with claim 10 in which said impregnant fully fills said pores.

14. In a photoconductive device in accordance with claim 10 in which said impregnant partially fills said pores. I

Notice of Adverse Decision in Interference In Interference No. 98,812 involving Patent No. 3,745,504, J. B. Mooney, IMPREGNATED POROUS PHOTOCONDUCTIVE DEVICE AND METHOD OF MANUFACTURE, final judgment adverse to the patentee Was rendered. Mar. 11, 1976, as to claims 8, 9, 10, 11, 12, 13 and 14.

[Oficial Gazette June %2, 1.976.]

Notice of Adverse Decision in Interference In Interference No. 98,812 involving Patent No. 3,745,504, J. B. Mooney, IMPREGNATED POROUS PHOTOOONDUOTIVE DEVICE AND METHOD OF MANUFACTURE, final judgment adverse to the patentee Was rendered Mar. 11, 1976, as to claims 8, 9, 10, 11, 12, 13 and 14.

[Ofiicz'al Gazette June 22, 1.976.] 

2. A method in accordance with claim 1 in which substantially the pores are fully impregnated.
 3. A method in accordance with claim 1 in which the pores are partially impregnated leaving some pore space unimpregnated.
 4. A method of manufacturing a phOtoconductive device comprising the steps of: depositing a predetermined quantity of solid glass impregnant upon a porous mass of sintered photoconductive material; and heating said photoconductive material and said impregnant to the melting point of said impregnant for flowing the molten impregnant into the pores of said porous mass.
 5. A method of manufacturing a sintered photoconductive device comprising the steps of: depositing a mixture including a particulate photoconductive material upon a first metallic electrode; heating said mixture to a first temperature to sinter said mixture to form a sintered photoconductive element having said first electrode in conductive contact with one side thereof; depositing a predetermined quantity of glass upon said sintered photoconductive element; and heating said glass and said sintered photoconductive element to a second temperature sufficient to melt said glass for flowing the molten glass into the pores of said sintered photoconductive element.
 6. A method of manufacturing a sintered photoconductive device as recited in claim 5 which further includes the step of providing a second metallic electrode in electrical contact with the other side of said sintered photoconductive element.
 7. A method of manufacturing a sintered photoconductive device as recited in claim 5 wherein the quantity of glass de-posited on the sintered photoconductive element is in excess of the quantity required to completely fill the pores of said element and which, upon being cooled, forms a layer of glass of a predetermined thickness upon said element and which further includes the step of providing a second metallic electrode upon said layer of glass, said layer of glass electrically isolating said second electrode from the other side of said element.
 8. A sintered photoconductive device comprising: a first metallic electrode; a sintered photoconductive element disposed in electrical contact with said first electrode; and an inorganic electrically nonconductive, solid impregnant disposed in the pores of said element.
 9. A sintered photoconductive device as recited in claim 8 which further comprises a second metallic electrode in electrical contact with said member and spaced apart from said first electrode.
 10. In a photoconductive device, including a sintered photoconductive element having pores, the improvement comprising an inorganic, electrically nonconductive, solid impregnant disposed in said pores.
 11. In a photoconductive device in accordance with claim 10 in which said impregnant is transparent to the radia-tion to which said element is sensitive.
 12. In a photoconductive device in accordance with claim 10 in which said impregnant is substantially inert to the ambient atmosphere and to said element.
 13. In a photoconductive device in accordance with claim 10 in which said impregnant fully fills said pores.
 14. In a photoconductive device in accordance with claim 10 in which said impregnant partially fills said pores. 